1. Field of the Invention
This invention relates to magnetic recording media of the coating type including a variety of magnetic tapes.
2. Prior Art
Needle iron oxide is typical of the magnetic powders used in magnetic recording media. However, since the needle iron oxide as such cannot accommodate a demand for increased magnetic recording density, a variety of magnetic materials having high coercivity and high saturation magnetization have also been developed.
For example, iron carbide system magnetic powders in which at least the surface is of iron carbide are proposed in Japanese Patent Application Kokai (JP-A) Nos. 71509/1985, 124023/1985, 184576/1985, 211625/1985, 212821/1985, 269225/1986, 85403/1987, 86537/1987, 86531/1987, etc. Also the inventors proposed a magnetic powder whose surface is composed mainly of carbon in Japanese Patent Application No. 272057/1991. These powders are characterized by high coercivity, high saturation magnetization, good electrical conductivity, and effective light shielding.
These prior art proposals, however, are insufficient in packing density and orientation of the magnetic layer because the dispersibility of magnetic powder in a binder is not taken into account. For example, some of the above-referred publications disclose to add fatty acids such as lauric acid and stearic acid to a magnetic paint as a dispersant, but such means alone is still insufficient in packing density and orientation.
Then the inventors proposed in Japanese Patent Application No.338016/1992 to increase the dispersibility of a magnetic powder whose surface is composed mainly of carbon or iron carbide by subjecting the magnetic powder to pre-treatment by kneading and dispersing the magnetic powder optionally with an anionic or ampholytic surfactant and a fatty acid, and using a resin containing an amino or ammonium salt group as a binder. However, gloss and surface roughness are still insufficient. There often occur coating defects such as longitudinal streaks. A further improvement in electromagnetic properties is also desired.